Practice, practice, practice. And then practice some more.
I assume you're already taking art classes in school, right? If so, talk to your instructor.
Take a trip to your local library and check out books on human anatomy, architecture, vehicles, etc. and practice by drawing what you see in the photographs in the books. There are also lots of instructional books at the library that will help teach you how to draw better. Or take a look at the Gnomon training DVD's. There are quite a few devoted to drawing.